Statics Question - Friction.

[golden]

A body, mass 25kg, rests in limiting equilibrium on a plane inclined at 30 degrees to the horizontal. If the plane is raised until it is at an angle of 60 degrees to the horizontal find:
(i) the least force acting up the plane required to keep the body in equilibrium;
(ii) the horizontal force required to prevent the body from slipping down the plane.

Drawing the first part to find μ (where you draw the sloping edge 30 degrees to the horizontal), it turns out that μ = 1/sqrt(3) or ~0.577.

But on the second drawing, (i), where the sloping edge is 60 degrees to the horizontal, why is the friction in the same direction as the force?
When a force is being applied in one direction, shouldn't friction oppose it going in the opposite direction?

Just for reference:
(i) = 14.43 kg wt
(ii) = 14.43 kg wt

[Yitzi_K]

Friction is not opposed to the force. It is opposed to the direction of motion. So even with a force taking it up the slope, if the gravitational forve will still be strong enough to take it down the slope, then the friction will be going up the slope as well as the force so as to keep it in equilibrium.

[98.40_for_sure]

Friction is only applicable to questions that involve "rough plane" or "coefficient of friction". Said friction is a force in the direction opposing the direction of the net force or resultant force to attempt to bring the body to equilibrium or rest. It does not oppose the direction of the force, for example; a body can still be in motion down an inclined plane with both a force + friction pulling the opposite way. The friction is not pulling it down opposing the force, but acts to attempt to slow the body down by pulling opposite to the NET force.

[theuncle]

Not exactly... as Yitzi said, the direction of friction is completely independent of any net force acting on the object, it acts to oppose the motion, which when the object starts at rest happens to be in the direction of net force (obviously!)